Thanks for taking the time to reach out to the QuickBooks Community. I hope you're doing well today.
In QuickBooks Online, the Make inactive option is likely greyed out because the clients have active, open transactions. Such as unpaid invoices, unbilled charges, or active projects linked to their profiles.
If this isn't the case for you, let's try some basic troubleshooting steps. Such as clearing the cache from your browser.
Web browsers collect cache to save time when loading repetitive data and images. Over time these files can become outdated and corrupted, causing issues like not having the option to make a client inactive. I'm including the links for all supported browsers that contain the steps to clear the cache below.